The long-term objective of this research is to understand the role of acyl-coenzyme A:cholesterol acyltransferase (ACAT) in human health and disease, particularly in atherosclerosis, a leading cause of morbidity and mortality worldwide. The current proposal has two main goals: to continue to conduct structure-function analysis of ACAT1 in vitro, and to examine the role of macrophage ACAT1 in vivo in the Apoe-/- mouse model and the Ldlr-/- apoB48-deficient mouse model for atherosclerosis. As a key enzyme in cellular cholesterol metabolism, ACAT utilizes cholesterol and long-chain fatty acyl coenzyme A to produce cholesteryl esters, and is allosterically activated by its own substrate, cholesterol, to guard against excess buildup of cholesterol at the endoplasmic reticulum. ACAT1 is the major isoenzyme of ACAT in macrophages and plays a key role in foam cell formation in the early stages of atherosclerosis. ACAT1 is also expressed in many other cell types, including hematopoietic stem cells in the bone marrow. The physiological roles of ACAT1 in various tissues remain to be carefully investigated. Thus, the aims of this project are 1) to identify key amino acid residues involved in substrate binding, catalysis, and/or allosteric control of ACAT1; and 2) to delineate the effects of inactivating macrophage Acat1 during the initiation and progression stages of atherosclerosis. We will employ a new procedure for expression and purification of the recombinant hACAT1 enzyme and seven different biochemical assays to pursue Aim 1, and will employ a newly developed macrophage-specific Acat1 KO mouse (Acat1-M/-M) model as a novel reagent to pursue Aim 2. ACAT1 is a potential target for treating Alzheimer disease and other diseases in humans. Most of the ACAT inhibitors currently available cause non-specific cytotoxicities and inhibit other enzymes with similar active sites, potentially causing other adverse side effects. The outcome of this application will help investigators to design ACAT1-specific inhibitors with minimal side effects, and help to determine the utility of ACAT1 as a target for treating atherosclerosis.